Many hydrocarbon resources (i.e., heavy crude oils and tar sand fluids) and residues (i.e., bitumen) are non-free flowing materials. Some of these resources are also found in remote areas of Canada and Alaska, requiring transportation over long distances (i.e., hundreds of kilometers). These viscous resources present significant challenges to economic recovery and transportation to a refinery and/or the consumer.
The high viscosity of many heavy crudes or other feeds make transportation by conventional pipeline methods nearly impossible without process treatment. Treatments are needed to change at least the viscosity of the transported crudes. Treatments have included dilution with lighter components such as condensate, transporting them in heated pipelines, and upgrading (e.g., partial thermal cracking or refining under elevated temperature and pressure conditions). However, all of these treatments are costly.
Lighter components for dilution may not be available or economic to transport to a remote site. Heating for long distances requires large insulation and fuel costs, especially in cold climates. Although upgrading can also decrease refining costs, it typically requires costly surface facilities, such as heavy wall pressure vessels, and requires continual operational expenditures. Because of these costs, the often remote location, large uncertainties of well production, and the finite life of productive wells, the economic development of many remote heavy crude reserves has not been accomplished.
An upgrading alternative to high temperature and pressure containing surface facilities (e.g., large, heavy wall surface vessels) is a Geotreater.TM. process, developed by Resource Technology Associates. The Geotreater.TM. process places a long, concentric tubular unit within a deep (e.g., approximately 1500 meters or 5,000 feet) borehole, adding heat under high downhole pressures to thermally crack viscous components and upgrade these feeds. The weight of down-flowing crude (near the bottom) replaces costly high pressure pumps.
The process chemically and physically alters (i.e., partially upgrades or thermally cracks) the pressurized feed stream to achieve a lower viscosity product. A downhole combustion make-up heat source is provided by an oxidizer supply pipe and downhole oxidizing reaction/combustion. A portion of the downhole heating transferred to the product stream is recovered by the down-flowing feed stream in the long tubular unit acting as a concentric tube heat exchanger. The earth around the borehole (i.e., formation) is also normally a good thermal insulator. This combination of regenerative heating (i.e., product stream heat transferred to feed stream), low pressure pumping requirements, and low thermal losses once the process is stabilized (approaching an adiabatic process except for make-up heat), result in minimal down-hole make-up heating and high efficiency. In pilot unit testing, reductions in viscosity of as much as two orders of magnitude, as well as increases in API gravity, pour point reductions, and increased residuum conversion were achieved without significant coking.
A critical control aspect of the Geotreater.TM. process is the downhole make-up heating (e.g., combustion). This must be accomplished in a manner which minimizes residence time (and resulting cost), but also avoids excessively rapid heating. Control of make-up heating can be accomplished by temperature based control of injected oxygen pressure and quantity. Injection and combustion typically occur in the unit after the down-flowing feed stream makes a U-turn near the bottom and begins its upward flow. Oxygen must also be carefully controlled to be well mixed to avoid hot spots.
The post-combustion temperature is controlled to avoid forming significant amounts of coke. Coke is a carbonaceous material which precipitates from feed, such as bitumen. Coke precipitation reduces the yield of high value refined hydrocarbon products and can cause scale, pumping, erosion and other problem which derive from the presence of a solid component in a flowing fluid.
The combustion heat and pressure upgrade the crude, but water, carbon dioxide, hydrogen and other combustion or reaction products are also formed in the product stream. Some of these combustion products may participate, further react, or otherwise assist in the upgrading, but combustion products may also destabilize the lower viscosity product stream.
Water may also be present in the crude oil source and/or may be added to the feed stream for temperature, pressure, and process reaction control. Although some of these addition/combustion products may be useful in upgrading the feed stream, excessive amounts (i.e., amounts which destabilize the crude oil product stream) must be removed prior to pumping and transport within a pipeline. Coke, sulfur, and other undesirable products in the discharge or product stream may also have to be removed to stabilize the product stream. Sulfur can be removed by a Claus process and tail gas unit. Post-combustion stabilization and combustion product removal are typically accomplished at on-site surface facilities. The stabilized product stream can then be economically transported and refined.
The necessity to stabilize and remove excessive water, gases, and other products adds significant capital and operating costs and risks to the Geotreater.TM. process. Since different feed streams require different heating and produce different combustion products, these removal facilities must also be sized for various quantities even if the feed stream throughput is fixed. Startup, process upset conditions, and non-uniform combustion further complicate the design and operation of these surface facilities. These factors can also create temperature control problems and result in excessive coking at critical areas.
None of the current Geotreater.TM. approaches known to the inventor eliminates the problem of significant on-site facilities to stabilize and remove excessive quantities of water, gases, and other products. In addition, combustion control problems and increased cost result from coking at critical areas.